Irregular Long‐RP Supraventricular Tachycardia Terminated by Double Ventricular Scan Pacing: What Is the Mechanism?

A woman in her 50s was referred to our hospital because of symptomatic palpitations. A surface 12-lead electrocardiogram showed a narrow QRS tachycardia with a long RP interval. At baseline, the atrio-His (AH) and His-ventricular (HV) intervals were 82 and 46 ms, respectively. A narrow QRS tachycardia was induced by ventricular burst pacing (Figure 1). The earliest atrial activation site was recorded at the proximal coronary sinus (CS). The tachycardia cycle length (TCL) was irregular and ranged from 360 to 400 ms. Ventricular overdrive pacing terminated the tachycardia. Ventricular single scan pacing did not terminate the tachycardia at a minimum coupling interval of 200 ms. Ventricular double scan pacing with a fixed sense–S1 interval of 220 ms reproducibly terminated the tachycardia as the S1–S2 coupling interval was progressively shortened in 10-ms steps (Figure 2). During scan pacing with progressive S1–S2 shortening, the AA interval immediately after the S1 stimulus was unstable, ranging from 360 to 390 ms. What is the mechanism of this tachycardia? An unstable AA interval after the first scan pacing with a fixed sense–S1 interval suggested that the S1 stimulus did not affect the tachycardia. Scan pacing with a short S1 coupling interval of 220 ms, which was short enough to capture the ventricle during the His non-refractory period, did not affect the tachycardia. This finding ruled out orthodromic reciprocating tachycardia (ORT) using an atrioventricular, nodoventricular, or nodofascicular bypass tract. S2 pacing terminated the tachycardia without atrial penetration, as short VA conduction was not observed during ventricular pacing at the initiation of the tachycardia. This ruled out atrial tachycardia. Therefore, the tachycardia was diagnosed as fast–slow atrioventricular nodal reentrant tachycardia (AVNRT). Cryoablation at the site of the earliest atrial activation successfully terminated the tachycardia and rendered it non-inducible. Truly irregular supraventricular tachycardia remains one of the challenges in making a precise diagnosis. The irregularity makes it difficult to interpret the results of entrainment pacing. Previous reports have demonstrated the usefulness of ventricular overdrive pacing and His-refractory ventricular extrastimulus pacing in the differential diagnosis of long-RP tachycardia, and a long post-pacing interval (PPI) minus TCL >125 ms has been accepted as a criterion favoring atypical AVNRT over nodofascicular or decremental atrioventricular ORT [1-3]. These methods rely on a stable and regular TCL for accurate interpretation and therefore could not be applied in the present case because of TCL irregularity. Moreover, considering the possibility of a bystander concealed nodoventricular or nodofascicular accessory pathway [4], continuous observation during ventricular pacing is mandatory. In the present case, we used fixed S1 and scan S2 pacing. Importantly, we aimed to confirm non-response to S1 by demonstrating a lack of reproducibility in the atrial response despite delivering S1 at an identical coupling interval. Furthermore, by fixing the sense–S1 interval at 220 ms, an interval at which the His bundle would be expected to be excitable, the non-response to S1 allowed us to exclude the involvement of an accessory pathway, either as part of the tachycardia circuit or as a bystander. The fixed sense–S1 interval maximized the likelihood of a non-response to S1 pacing, and reproducible termination by S2 pacing without atrial penetration was observed. This case highlights the importance and benefit of increasing the number of scan pacing attempts to better interpret the response to pacing and help achieve a more accurate diagnosis of irregular tachycardia. The authors have nothing to report The authors declare no conflicts of interest. This case report was approved by the Ethics Committee of Kyoto Tanabe Central Hospital (Approval No. 2025-014). The data that support the findings of this study are available from the corresponding author upon reasonable request.